The present invention relates to a method for the preparation of a cis-alkylcyclohexanol or, more particularly, to a method for the preparation of an alkylcyclohexanol by the catalytic hydrogenation of the corresponding alkylphenol with hydrogen in which the yield of the cis-isomer relative to the trans-isomer is markedly increased.
Alkylcyclohexanols are useful compounds as an intermediate for the synthesis of various kinds of perfumes, medicines and other organic chemicals. Of the two stereoisomers of the alkylcyclohexanols, viz. the cis- and trans-isomers, the cis-isomers are particularly useful as a perfume as such or as the intermediate compound for the synthesis of perfumes. Therefore, one of the more important problems in the technology of organic synthesis is to obtain an aklylcyclohexanol product rich in the cis-isomer.
Alkylcyclohexanols are prepared most conveniently by the catalytic hydrogenation of the corresponding alkylphenols with hydrogen. Several methods have been proposed as directed to the enhancement of the cis/trans selectivity, among which is known a method utilizing a rhodium catalyst, e.g. metallic rhodium, rhodium-platinum and rhodium-ruthenium, supported on certain catalyst carriers such as carbon and the like (see, for example, Japanese Patent Publication No. 42-13938 and U.S. Pat. No. 2,927,127). This method of rhodium-catalyzed hydrogenation is disadvantageous, not only because of the expense of using rhodium, but also because the poor selectivity with respect to the cis-isomers. The selectivity problem is particularly troublesome when the hydrogenation reaction is undertaken at an elevated temperature in order to increase the reaction velocity. Furthermore, when the catalyst is to be used repeatedly in numbers of runs, it has been found that the catalyst is not sufficiently durable.
In addition, the reaction products obtained by the rhodium-catalyzed hydrogenation of alkylphenols contain large amounts of alkylbenzenes as the decomposition products of the hydrogenation reaction as well as considerable amounts of unidentified impurities with consequent low yields of the desired alkylcyclohexanols based on the starting alkylphenols. There is also difficulty in subsequent purification, especially, when the alkylcyclohexanols are intended to be used in perfumery so that the rhodium-catalyzed hydrogenation is somewhat questionable industrially.
Another approach for improving the cis/trans ratio of the alkylcyclohexanols is the use of ruthenium catalyst as disclosed in Maruzen Sekiyu Giho (Technical Bulletin of Maruzen Petroleum Co.), No. 16, pp. 77-87 (1971), according to which the ruthenium catalyst is used in a form of ruthenium oxide or a ruthenium catalyst supported on a carbon carrier. This method of ruthenium-catalyzed hydrogenation gives an improved cis/trans ratio in comparison with the rhodium-catalyzed hydrogenation reaction but the durability of the catalyst activity is still unsatisfactory necessitating frequent use of fresh catalysts. It is also desirable to decrease the amount of alkylbenzenes as a by-product contained in the hydrogenation products obtained with the above mentioned ruthenium oxide or ruthenium/carbon catalyst for the method to be of practical value in the technology of perfumery.